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New guy, Odd shapes

2062 Views 1 Reply 2 Participants Last post by  fbov
So, what with a number of elements out of my way (stupid work), I am about to re-enter/ re-build my listening lab and start again with this project.
Before I begin, though I wonder if y'all might have some advice...
Months ago I ran a number of tests (using REW) with different drivers trying to optimise the various elements like Vb, port volume and XO values... My results, however were discouraging. Not that they sounded bad... but they sounded the same... Drastic changes in volume and port size, even sealing the port, produced what appear to be negligible changes in the response curve.:dontknow:

I'm running an ecm8000 on a Art USB preamp, via my MacBook. Doing nearfield measurements at 1m.

Now, I have yet to get deep enough into the software to know if there were any results of note in things like phase response, or the like, and subjective listening is still a ways off, but I was wondering if there is something I'm missing. Halving the volume of the box makes no real difference, doubling the port length has about the same effect as blocking the port. What gives?

I know I'm short on some details here, and I'll add graphs once the lab is restored, but if anybody can see through this haze and suggest whatever it is I might be missing, It should save me a bunch of time in this test phase.

Thanks in advance.
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No expert here, but normally, a ported box has to be measured twice, once near field (and I mean near field, like 0.01m, not 1m) to capture actual driver response, and once in the port, then merged. Otherwise the room plays too large a part. Alternatively, assuming port and driver face the same way, ground plane measurements will give you an anechoic response.

Next is to set expectations - tuned boxes aren't that sensitive to perturbations around a good center point. I did a sensitivity analysis some time ago, looking at the affect on tuning frequency of several perturbations. Box size and port size are strong drivers, port length and port end correction, not so much. 50% change in port length should be barely audible, even if it's quite measureable.

If you want large changes, play with the XO components that adjust driver level, padding resistors, L-pad, whatever's used to match driver levels. I can easily hear changes in tonal balance. I can also easily hear a tweeter that's crossed too low - high harmonic distortion.

Part of your listening lab projects should be calibration of the listener. All people are not equally sensitive to all aspects of sound reproduction. I'm sensitive to HD and tonal balance; you may notice timbre I don't. We're all different, and Harman has a great resource in this regard.

Have fun,


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